1. Field of the Invention
This invention relates generally to apparatus for dispensing wallboard compound to a wallboard tape applicator and particularly relates to a gravity-fed screw-driven positive displacement pump for delivering semi-liquid pastey wallboard compound to a wallboard tape applicator within an automatically controlled pressure range.
2. Description of Prior Developments
Pressurized systems for feeding wallboard compound to a wallboard applicator or taping head have been in use for many years. Such systems typically include a pressurized reservoir containing a viscous liquid or semi-liquid paste known as wallboard compound or "mud". The wallboard compound is generally dispensed under pressure from a lower or bottom region of the reservoir while the upper region of the reservoir is maintained at an elevated pressure to provide the required driving force. A volume of compressed air is generally used to provide this driving force.
Examples of prior compound feeding systems are disclosed in U.S. Pat. No. 3,007,837 issued to Goode, Jr., U.S. Pat. No. 3,707,427 issued to Erickson, and U.S. Pat. No. 4,080,240 issued to Dysart. Although prior pressurized compound feeding systems have been designed, few, if any, have met with full acceptance by the construction trade. A particularly bothersome problem associated with prior wallboard compound feeding systems has been their inability to maintain a relatively constant flow of compound to the applicator taping head.
More particularly, air pressurized systems require the entire volume of wallboard compound to be pressurized under a compressed volume of air. These systems generally experience a significant hesitation or time lag between the time when the compound is initially released through the applicator head and the time when the flow of compound reaches a steady state flow rate. This hesitation is attributable to the spring-like action of the compressed air and the inertial resistance to movement presented by a large volume of static wallboard compound.
The result of this hesitation is an uneven application of wallboard compound through the applicator resulting in inadequate or excess compound being applied to the wallboard being taped. More particularly, because the entire volume of compound is driven by the spring-like force of a compressed volume of air, movement of the compound through the feed and applicator system is not easily controlled or maintained within predetermined flow rates. Moreover, because of this delayed response of compound to the applicator, the operator who moves the applicator head with an uneven motion will often produce an uneven layer or thickness of compound on the wallboard. This condition often requires time-consuming touchup work and is not appreciated by operators who often work on a piece rate schedule.
Accordingly, a need exists for a wallboard compound feed system which provides a steady and closely-controlled flow rate of compound through a wallboard compound applicator. A further needs exists for a positive displacement compound feed system which experiences negligible hesitation or lag time between the initial release of compound through the applicator and the time at which the compound reaches its steady state flow rate. Another needs exists for such a system which does not require significant clean-up of the work area due to the application or leakage of excess compound from a wallboard compound applicator.